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Topic: SLA Battery: charging advice (Read 2707 times)

Hello, I was going to purchase an SLA battery that I intend to recharge whenever it runs out, and I was hoping someone could tell me if I'm buying the right components, and if there is anything I should watch out for when recharging them.

I was going to purchase an SLA battery that I intend to recharge whenever it runs out,

Some words of warning here...SLA's are not too keen on being deep cycled, but they are cheap (and heavy) compared to Lithiums.To give an SLA the longest possible life, charge it before it runs all the way down (don't top it off repeatedly though)An end of discharge circuit that cuts power is not a must like in lithiums, but will give the SLA better terms to work on.If you for some reason has to store it for a long time, recharge at least once each 2..3 month, or it will build sulphate on its plates.

and I was hoping someone could tell me if I'm buying the right components, and if there is anything I should watch out for when recharging them.

You can build the charger yourself - that way you know how it behaves.I just made a charger with auto cut-off at full charge for a 12V/4Ah SLA belonging to a friend and, if you're interested, I could translate the (Danish) comments.The Auto cut-off part of the circuit could be removed, if it has to be as cheap as possible, but I won't recommend it though.

I'd go for the battery alone and make the charger and the cables to suit.An SLA needs to be charged at between 20% and 30% of it's capacity (1.4A to 2.1A for your 7Ah), so you need a charger giving more than 1A anyway.Might be a good idea getting the narrow spade terminals that fits the battery terminals and, as they can be hard to get on and off (which is loosening them, giving less contact), install a cable that have a connector for charging and goes to the load at the same time - then it can be charged directly, or removed with less trouble.

I have never charged a battery like this before, and I was hoping someone could tell me if I have chosen an incorrect piece of equipment or if there are any dangers or anything I should be aware of.

Dangers: Charging a lead-acid battery (like an SLA is) makes it split water into hydrogen and oxygen and that is very explosive, so no sparks near it when charging and immediately after - allways cut the power on the wall socket first.And don't short circuit it either. It packs a generous *kaboom*.

Hi! For a 12V 7amp,a 0.8 charger is enough, but it will take 7 hours. You can use smart charger like a CTEK, wich has a disulphation cycle for a longer life.SLA hate being completely flat, they can last up to half the time, wich is about 7 years normally.Use a permanent connention to the battery and use an other connector far away from it. Always pull the power cord from the plug before connecting the charger to the battery to avoid sparks...And explosion!SLA looses 25% of capacity every 6 month with normal temperature

Thank you again, Soeren! If possible, I would really like to see what you built for your friend. If you think it might lead to a safer and more efficient way of charging the SLA battery, I'll try my best to mimic whatever you did. I really appreciate your help!

Hi! For a 12V 7amp,a 0.8 charger is enough, but it will take 7 hours. You can use smart charger like a CTEK, wich has a disulphation cycle for a longer life.SLA hate being completely flat, they can last up to half the time, wich is about 7 years normally.Use a permanent connention to the battery and use an other connector far away from it. Always pull the power cord from the plug before connecting the charger to the battery to avoid sparks...And explosion!SLA looses 25% of capacity every 6 month with normal temperature

Thank you, french_guy! And thank you for giving me a picture to go along with your advice, it really helps. All this talk of exploding batteries makes me very nervous, so I'll try my best to make sure everything is safe in my design.

Sorry, but no!Modern SLA's needs to be charged with between 20% and 30% of the capacity, i.e. 1.4A to 2.1A for the battery in question, if you want them to live as long as possible (7 years is not possible, unless you are satisfied with a ridiculously low capacity).The high current only last a relatively short time in a taper charger, but this "high" current helps against sulfation.

There's a graph showing the approximate charge curve here:

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I would really like to see what you built for your friend. If you think it might lead to a safer and more efficient way of charging the SLA battery, I'll try my best to mimic whatever you did. I really appreciate your help!

The graph posted by frenchguy shows the desulphation phase before the CC/CV phases of charging. The retail chargers i've played with only trigger the desulphation after the battery is charged. Which way is correct, or does it not really matter? Also, does desulphation work or is it more of a gimmick?

If the plates have sulphation then the battery will not take a full charge so I believe desulphation should be done first. It can help in extending a batteries life and works if the sulphation is not too extreme.

If the plates have sulphation then the battery will not take a full charge so I believe desulphation should be done first. It can help in extending a batteries life and works if the sulphation is not too extreme.

Depending on the state of charge, it would be prudent to at least charge to whatever the battery will take before the desulphation phase, as it's supposed to get a series of resonances going in the battery and they should allegedly increase in power (while voltage is going down). A final charge might be in order.A full desulphation phase can, allegedly, take months to get the full effect, if the battery is severely sulphated.

That's assuming desulphation does anything usable at all. The World is still without any conclusive scientific evidence on the effects of desulphation and until such time, I'll consider it a "religious" stand/a matter of belief, working just as placebo medicine - if you believe it works, you'll scour up some "evidence" to substantiate it.

A good strong charge, within the 20% to 30% (not running it completely flat and recharging soon after a discharge) with the general form shown in the graph in my previous post, is what is generally recommended to keep an SLA in working order.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I would really like to see what you built for your friend. If you think it might lead to a safer and more efficient way of charging the SLA battery, I'll try my best to mimic whatever you did. I really appreciate your help!

If your relay are different, I can change the PCB to match (if you give me the details).

The regulator should have a transfer heat sink mounted between the PCB and the regulator itself.Any questions about it?

Wow, thank you for giving me such a detailed schematic! And thank you for offering to explain it. I will try to learn from it before I ask any questions, and I will try to figure out how to build it or something similar for myself. I am no good at PCB design (I barely even have experience soldering!), but having a schematic like this helps me immensely. Thank you again, Soeren!

Hi! For a 12V 7amp,a 0.8 charger is enough, but it will take 7 hours. You can use smart charger like a CTEK, wich has a disulphation cycle for a longer life.

Just took a closer look at this charger - Don't buy it - Rubbish Alert!

The datasheet claims it to be able to to charge a 60Ah battery to 80% of capacity in 60 hours - with 0.8A!With the loss in charging being around a third (you have to "spend" around 150% of the capacity on charging)Darned close to an over-unity-machine!

A 60Ah battery charged with 0.8A will sulphate the plates and 0.8A "desulphation" pulses will do nothing to save the battery from an early demise. That battery (for its short life) will take 60Ah/0.8A*1.5*80% = 90 hours to bring to 80%The charger doesn't differentiate between lead-acid battery types, although its common knowledge that they need to be treated differently and it claims to fit batteries from 1.2Ah to 32Ah (or to 60Ah, depending on where you read it and up to 100Ah for floating) with the same 0.8A

The rest of the datasheet is so full of nonsense as well, so I'll advice against wasting money on it.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I am no good at PCB design (I barely even have experience soldering!), but having a schematic like this helps me immensely. Thank you again, Soeren!

You're welcome If you build it on perf-board or similar, keep the contact(s) of the relay and the "go-switch" (if auto-off is used) a long way from the low voltage parts and make sure they're not able to short something even if a lead break loose!The first and foremost rule of construction is: Don't get killed!

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I am no good at PCB design (I barely even have experience soldering!), but having a schematic like this helps me immensely. Thank you again, Soeren!

You're welcome If you build it on perf-board or similar, keep the contact(s) of the relay and the "go-switch" (if auto-off is used) a long way from the low voltage parts and make sure they're not able to short something even if a lead break loose!The first and foremost rule of construction is: Don't get killed!

I'm definitely going to follow your precautions if I build it! I'm relieved I didn't try to build something before you posted! I'll do my best to stay alive! Thank you!

Hi, I'm sure you all know what you are talking about, I just wanted to say I' m using a CTEK 0.8 amp on a 12v 12Amp/h power-sonic for 6 years now, for 2 lights and a water pump, and it's still ok. Now I'm scared it lets me go any minute!Thanks for letting me know, I might change the charger for next battery

Well, it takes about 5 hours to charge, but I never wait untill its flat before I charge it.

5 hours of 800mA max. (assuming the charge was the full 800mA for the entire charge) equals around 2.7Ah of stored charge, due to the electrical and chemical loss in the process). Were we to include the tapering off phase near the end, it will be even less. Perhaps the battery is really a bit too large for your use then.

If you measure the voltage of the battery (directly at the terminals with nothing else connected) fully charged and then left untouched (neither charged nor discharged) for 6 to 8 hours and then again after a relatively light discharge, like ~500mA during ~5 minutes, you'll get a not a direct measure, but a fairly good impression of the condition of the battery.

First measure should be around 12.65V or better and the difference between the 2 measures will show an inverse relationship to the state of the battery. I cannot tell you what exactly the measure should be the second time, as batteries are different (this method works best if you do it from when the battery is new), but it shouldn't get much under 12.60V if the battery is good.

Don't worry too much though, the usual failure mode of lead-acid in regular use is a slow decay. This can be catastrophic in a car at winter times, where the starter may pull the voltage so low that there isn't power to the ignition, but used as a power supply, you'll just experience a shorter runtime and since you seemingly never taps the full power anyway, your battery may live to the point where the acid has eaten away the plates more or less.

Lots of ham radio operators use a regular car battery to buffer a small supply, some of the really weird audio buffs runs their Hi-Fi off batteries as they claim they can otherwise hear frequency deviations at the mains generators of the electricity plant (don't ask for my opinion of their sanity ) and I used to run a 60Ah car battery (enclosed in a plastic jug to avoid acid splatter) through a resettable mains fuse for years, when I was most active designing car electronics, so as long as you are aware of how to handle it and are not too sad if it dies - always use a suitable fuse though, even if it's a relatively small capacity. A 50Ah battery shorted with eg. a heavy screw driver will melt the blade and probably crack/explode and splatter its vicinity with acid electrolyte.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives